This is a project I personally don’t have the guts to do, as I’m bit shy (that’s the whole reason why I’m behind the lens to begin with!), but I know some people doing this project and I’m loving their results so far. The idea is to approach someone you’ve never met before, ask if you can take their picture, and talk to them a bit to get some kind of backstory, and then upload the photo (and a short description of who they are and what you talked about) to flickr and add it to the 100 Strangers group. Then repeat 99 more times.
Not only will get photos of different people in your collection, but you just might make a new friend or two. You can, of course, skip the flickr part and do this project on your own, but you would gain a lot of inspiration from the group.